1. Field of the Invention
The invention relates generally to the field of fluid velocity measurement and, more particularly, to a method and system for measuring the velocity of fluids by way of hot wire anemometry.
2. Description of Related Art
Many processes and devices make use of moving fluids. Some examples are heating and ventilating systems that make use of moving air, as well as manufacturing processes that rely on the use and flow of various gasses. Typically, for these processes and/or devices, it is important that the velocity of the fluid be accurately determined. Accordingly, various techniques to determine the velocity of a fluid have been developed.
One such technique is commonly referred to as hot wire anemometry. Generally, hot wire anemometry involves heating an object (or element) such as a temperature dependent resistor (e.g., thermistor or hotwire), to an elevated temperature, and placing the heated object in the path of a fluid flow and measuring the rate that energy is removed from the object by the fluid. Because the velocity of the fluid is an important component of the cooling effect of the flow, the velocity of the fluid can be determined mathematically once the rate of energy removal from the object is determined. As such, the amount of heat removed from the object due to the flow of the fluid is related to the fluid velocity and other parameters (such as the fluid temperature).
When implementing a system to measure fluid velocity via hot wire anemometry, there are three common methods for maintaining the elevated temperature of the heated object; constant current, constant temperature and constant differential temperature. The constant current method involves maintaining a constant current at the heated object, while the constant temperature method involves maintaining a constant temperature at the heated object. Consequently, both the constant current and constant resistance techniques rely upon the solving of relatively complex equations to determine the velocity of the fluid.
The constant differential temperature technique includes maintaining a constant temperature differential or difference between the heated object and some other temperature, such as the ambient temperature of the measured fluid.
An advantage of the constant temperature differential technique is that it completely removes the temperature variable from the velocity calculation, because only the temperature difference between the object (or element) and the fluid are significant. A disadvantage with this technique, however, is that it is typically relatively difficult and complicated to implement because the controlled (servo) parameter is neither resistance (as in the constant temperature approach) nor current (as in the constant current approach), but temperature, which needs to be converted to a measurable electrical quantity. Accordingly, an object (or element) may be used, such as a thermistor, or hot wire, wherein resistance (an electrically measurable quantity), varies in relation to temperature.
Typically, however, objects whose resistance varies as a function of temperature, such as thermistors and hot wires, have a temperature/resistance relationship that varies from sensor to sensor and is relatively non-linear, thus complicating the use of resistance as the measurable parameter. Accordingly, the use of constant temperature differential techniques for implementing hot wire anemometry is typically relatively cumbersome, prone to inaccuracies and inefficient. As such, a need exists for an improved method and system for measuring the velocity of fluids.